Albert Barnes Commentary Ezekiel 9:4

"And Jehovah said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry over all the abominations that are done in the midst thereof." — Ezekiel 9:4 (ASV)

Mercy precedes judgment. So in the case of Sodom (Genesis 19:0), and in the last day (Luke 21:18), (Luke 21:28); (Revelation 7:1). This accords with the eschatological character of the predictions in this chapter (see the introduction to Ezekiel).

A mark - literally, “Tau,” the name of the last letter of the Hebrew alphabet. The old form of the letter was that of a cross. The Jews have interpreted this sign variously; some considered that “Tau,” being the last of the Hebrew letters and so closing the alphabet, denoted completeness, and thus the mark indicated the completeness of the sorrow for sin in those on whom it was placed.

Others also observed that “Tau” was the first letter of Torah (“the Law”) and that the foreheads were marked as a sign of men obedient to the Law. Christians, noting the resemblance of this letter in its most ancient form to a cross, have seen in this a reference to the cross with which Christians were signed. The custom for pagan gods and their devotees to bear certain marks furnishes instances where God was pleased to employ symbolism, commonly in use, to express higher and more divine truth. The sign of the cross in baptism is an outward sign of the designation of God’s elect, who at the last day will be exempted from the destruction of the ungodly (Matthew 24:22), (Matthew 24:31).